- "As an immortal being, I'm afraid only you can give up your life."
- —The Sea Hag to mermaid Mylie.
Immortals are beings incapable of dying of natural means, the opposite of mortals. However, they can still be killed or vanquished through unnatural means like magic or physical harm.
Although beings can be born immortal, some acquire immortality through other means. Immortal beings are often immune to the effects of aging or at least age at a slower rate as mortals.
- One way of reaching immortality is by obtaining a large amount of magical power.
- Drinking the water from the Eternal Spring grants immortality.
- Mermaids cannot die unless they willingly give up their immortality and are capable of giving it up and passing it on to others.
- The Angels of Destiny and the Angel of Death play a vital part in the Grand Design. For this reason, it is believed they are true immortals.
- Barbas could be considered a true immortal as well, as according to him "fear never dies".